1. Field of the Invention
This invention relates to the use of magnetic tunnel junction (MTJ) configurations to form tunneling magnetic read heads (TMR read heads). In particular, it relates to the formation of a low resistance MTJ by use of an under-oxidized tunneling junction layer that is naturally oxidized and formed on an electrode with an oxygen surfactant layer.
2. Description of the Related Art
The magnetic tunnel junction (MTJ) basically comprises two electrodes, which are layers of ferromagnetic material, separated by a tunnel barrier layer, which is a thin layer of insulating material. One of the electrodes, the “pinned” electrode, has its magnetic moment fixed in spatial direction by an adjacent pinning layer (typically a layer of antiferromagnetic material). The other electrode, the “free” electrode, has a magnetic moment that is free to move in the presence of external magnetic fields. Although the tunnel barrier layer between the free and pinned electrodes is an insulator, it is sufficiently thin so that there is a small but finite probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. Thus, a probability dependent current can flow between the free and pinned electrodes. The tunneling probability is spin dependent, depending on the orientation of the conduction electron spin relative to the magnetization direction of each of the ferromagnetic layers. Thus, if these magnetization directions are varied relative to each other, the tunneling current will also vary as a function of the relative directions for a given applied voltage. When used as a read head, the magnetization of the free electrode varies with the external magnetic field of the recorded medium and the resulting change in current is sensed by appropriate circuitry. The MTJ is already being successfully used as a read head. Fontana, Jr. et al (U.S. Pat. No. 5,901,018) disclose a MTJ magnetoresistive read head with a free magnetic layer that also functions as a flux guide to direct magnetic flux from the recording medium to the tunnel junction. The tunnel barrier layer taught by Fontana is formed by depositing and then plasma oxidizing a 0.5-2 nm Al layer to form a layer of Al2O3. Wang et al. (U.S. Pat. No. 6,462,541 B1) discloses sensing arrangment formed of a plurality of magnetic field sensors, each one generally formed of two magnetically permeable films separated by an intermediate layer of non-magnetic material. Mao et al. (U.S. Pat. No. 6,411,478 B1) teaches the formation of an MTJ type read sensor whose layer structure is horizontal rather than vertical so as to achieve a thinner overall fabrication.
For a read head to operate successfully with recording densities greater than 100 Gb/in2, it should have the following properties:                a) Very low junction resistance R for a given junction area A, (the product, RA, being less than or equal to 1 Ωμm2)        b) High ratio of dR/R (>10%)        c) High dielectric breakdown voltage (>0.5 volts)For a typical junction area: A=0.01 μm2, R must be approximately 100 Ω for the tunneling magnetoresistive junction (MTJ). For reference, R for a typical GMR sensor (not an MTJ) of this size is approximately 50 Ω.        
It is known that ultra-low junction resistance can be obtained only with an ultra-thin tunneling barrier. Al2O3 is known to be an insulator with a relatively wide band gap even when formed as an ultra-thin layer less than two atomic layers thick (<6 angstroms). Tunneling junctions using this material have been formed by a process of naturally oxidizing a thin layer of aluminum. In this regard, Gallagher et al. (U.S. Pat. No. 6,226,160) teach a method of thermally oxidizing a layer of Al between 5-9 angstroms thick deposited on a fixed ferromagnetic layer, which does not oxidize the surface of the fixed ferromagnetic layer on which the oxide layer is formed. HfAlOx has also been shown to have very low resistance at thin layer formations and the natural oxidation of a thin layer of HfAl has produced an RA as low as 0.6 Ωμm2 and dR/R of approximately 10%. Of the three requirements, a), b) and c), mentioned above, only adequate breakdown voltage has not yet been obtained and is not disclosed in the cited prior art.